The present invention relates generally to the field of optical amplifiers and lasers. More particularly, the present invention relates to a method and apparatus for providing high power pulsed laser sources useful for various applications including industrial applications such as trimming, marking, cutting, and welding. Merely by way of example, the methods and systems of the present invention have been applied to broaden the apparent laser linewidth in order to reduce Stimulated Brillouin Scattering. But it would be recognized that the invention has a much broader range of applicability.
Conventional laser-based material processing has generally used high peak power pulsed lasers, for example, Q-switched Nd:YAG lasers operating at 1064 nm, for marking, engraving, micro-machining, and cutting applications. More recently, laser systems based on fiber gain media have been developed. In some of these fiber-based laser systems, fiber amplifiers are utilized.
Some optical amplifiers and lasers utilizing a fiber gain medium are optically pumped, often by using semiconductor lasers pumps. The fiber gain medium is typically made of silica glass doped with rare-earth elements. The choice of the rare-earth elements and the composition of the fiber gain medium depend on the particular application. One such rare-earth element is ytterbium, which is used for optical amplifiers and lasers emitting in the 1020 nm-1100 nm range. Another rare-earth element used in some fiber gain media is erbium, which is used for optical amplifiers and lasers emitting in the 1530 nm-1560 nm range.
The wavelength of the optical pump source used for ytterbium-doped fiber amplifiers and lasers is typically in the wavelength range of 910 nm to 980 nm. The wavelength of the optical pump source used for erbium-doped fiber amplifiers and lasers is typically in a wavelength range centered at about 980 nm or about 1480 nm.
Because optical fibers usually have small diameters, they tend to be prone to optical nonlinear effects degrading the amplifier performance, especially in high peak power pulsed operation. The nonlinear effect threshold depends on the optical intensity, which is given by the ratio of the peak optical power to the optical beam cross-section. Since, in a number of optical fibers, the beam cross-section is small, the optical peak power to reach the nonlinear threshold can also be small. Optical nonlinear effects can also be present in bulk optical amplifiers using, for example, rods as the gain media, but since the beam diameter is much larger than in optical fibers, the optical peak power to reach the nonlinear threshold is likewise much larger in bulk optical amplifiers than in optical fibers.
One such optical nonlinear effect that has been observed to limit the output power in an optical fiber amplifier is Stimulated Brillouin Scattering (SBS). High intensity pulses in optical fibers generate high frequency sound waves, which periodically modify the index of refraction of the fiber, hence creating a moving Bragg grating. Bragg gratings can be used to couple different beams of light, one to another. In the particular case of SBS, the effect of the moving Bragg grating is to couple the incident signal light to a counter-propagating wave having a frequency down-shifted by the frequency of the sound wave, which is typically ˜10 GHz. Wave coupling results in energy being exchanged between the two waves. If the intensity of the incident wave is high enough, SBS in effect enables it to provide a very substantial amount of gain to the counter-propagating wave. In real-world high-power fiber lasers, the SBS mechanism can transfer large amounts of energy to a counter-propagating wave seeded only by Rayleigh scattering or amplified spontaneous emission, limiting the amount of amplification in the signal wave that can be extracted.
Thus, there is a need in the art for improved methods and systems related to high peak power fiber-based amplifiers.